Conventional metal bicycle frames, such as shown in FIG. 1, have long been known and widely used. Since the late 1800s, regardless of the type of metal used, bicycles have generally followed the same format: head tube 1, top tube 2, down or bottom tube 3, seat tube 4, seat stays 5, bottom bracket 6, chain stays 7, and bracket 8 connected in two triangles, forming the two-triangle, “diamond frame” most recognizable as a bicycle. Variations on this model have always included a seat tube or something similar extending between the top and bottom tubes to support the rider's weight. “Step-through” frames modify the diamond design, but still rely on a seat tube. Even suspension-type bicycles include some variation of the recognizable extended seat tube.
A bicycle frame allows for almost no vertical deflection. A large portion of this stiffness is carried through the seat tube, which experiences infinitesimal flex in a vertical plane. Under force, a bare frame will remain nearly rigid. The total frame deflection on a complete bicycle under vertical force will only be a small percentage of the flex available in the bike “system,” including tires, wheels, handlebars and saddle. The seat tube, then, is an expedient method of engineering a vertical-force-resistant frame.
The diamond frame, with its seat tube-dependent design, also serves to absorb vibrations and impact from the road and transfer them away from the rider. The impact force is redirected around the triangles and focuses the forces on the main tube welds more as compression than flex.
The traditional bicycle-frame seat tube also helps to reduce horizontal deflection, including lateral flex. Frames that experience lateral flex may have resultant difficulties in the bicycle's drive train (cog, chain ring, chain, derailleurs), brakes and overall stability.
Diamond-frame construction and its variants limit frame construction to the standard two-triangle format, which includes a seat tube. This reliance on building traditional-geometry bicycles defeats frame innovation, as well as any attempts to minimize material use.